KCNE1 Separates the Main Voltage Sensor Movement and Channel Opening in KCNQ1/KCNE1 Channels

Abstract

The IKs channel is a slowly activating potassium channel that generates one of the potassium currents that limits the duration of the cardiac action potential. The IKs channel consists of 4 pore forming alpha subunits (KCNQ1) and 2-4 beta subunits (KCNE1). The four KCNQ1 subunits form functional potassium channels, but the KCNE1 beta subunit is necessary to recapitulate the slow activation kinetics of the IKs channel. The mechanism by which KCNE1 slows the kinetics of KCNQ1 channels is a matter of current controversy. Here, we use a combination of voltage clamp fluorometry (VCF) and gating current measurements to show that IKs channel activation occurs in two steps: (1) mutually independent voltage sensor movements in the four KCNQ1 subunits generate the main gating charge movement and underlie the delay in the activation time course of the KCNQ1/KCNE1 currents, (2) a slower and concerted conformational change of all four voltage sensors and the gate, which opens the KCNQ1/KCNE1 channel. Gating currents develop with a similar time and voltage dependence as the first fluorescence component, as if the first component reports on the main S4 charge movement. In contrast to other Kv channels, the voltage dependences of the main voltage sensor movement and channel opening are separated by over 100 mV. The two activation steps in KCNQ1/KCNE1 channels can be farther separated by a disease-causing mutation in KCNE1. We determine rates and voltage dependence of the gating transitions to construct a model for KCNQ1/KCNE1 channels. Our model is consistent with that KCNQ1/KCNE1 channel has a fast S4 movement at negative voltages that moves the majority of gating charge and a slower second conformational change at positive voltages that moves a smaller amount of gating charge and opens the gate.